Transforming growth factor-β superfamily proteins represent a large family of cytokines that includes the TGF-βs, activins, inhibins, bone morphogenetic proteins (BMPs) and Mullerian-inhibiting substance (MIS) (for review, see Massague et al., Trends Cell Biol. 7:187-192, 1997). These proteins contain a conserved C-terminal cystine-knot motif, and serve as ligands for diverse families of plasma membrane receptors. Members of the TGF-β family exert a wide range of biological effects on a large variety of cell types. For example, they regulate cell growth, differentiation, matrix production and apoptosis. Many of them have important functions during embryonal development in pattern formation and tissue specification; in the adult, they are involved in processes such as tissue repair and modulation of the immune system.
Activities of the TGF-β superfamily proteins are regulated through various means. One of the negative regulations for the BMP subfamily of proteins is through a relatively large family of so-called Bone Morphogenetic Protein (BMP) antagonists/repressors. These BMP repressors represent a subgroup of proteins that bind BMPs, and interfere with BMP binding to their membrane receptors, thereby antagonizing their actions during development and morphogenesis.
The BMP repressors can be further divided into three groups of proteins based on structural analysis, especially the number of structurally conserved Cys residues in their C-terminal characteristic “Cys-knot” structures: the 8-, 9-, or 10-member ring Cys-knot BMP repressors. The 8-member ring (CAN subfamily) repressors can be divided further into four subgroups based on a conserved arrangement of additional cysteine residues—gremlin and PRDC, Cerbertis and coco, and DAN, together with USAG-1 and sclerostin. Orthologs of these human BMP antagonists in the genomes of several model organisms have also been identified, and their phylogenetic relationship has been analyzed (Avsian-Kretchmer and Hsueh, Mol Endocrinol. 18(1): 1-12, 2004. Epub 2003 Oct. 2, incorporated herein by reference).
Myostatin, or growth/differentiation factor 8 (GDF-8), also belongs to the transforming growth factor-β (TGF-β) superfamily (McPherron et al., Nature 387:83-90 (1997)). The human myostatin gene has been cloned (Nestor et al. Proc. Natl. Acad. Sci. 95:14938-43 (1998)), and it has been reported that myostatin immunoreactivity is detectable in human skeletal muscle in both type 1 and type 2 fibers. With respect to function, myostatin may play a role in negatively regulating the growth and development of skeletal muscle (Nestor et al., supra).
A study with myostatin knock-out mice provided the first evidence that myostatin is a key negative regulator of muscle development (McPherron et al., Nature 387:83-90 (1997)). In the myostatin null mice, the animals were significantly larger than wild-type mice and had a large and widespread increase in skeletal muscle mass. Furthermore, two breeds of cattle, characterized by increased muscle mass, have mutations in the myostatin coding sequence (McPherron et al., Proc. Natl. Acad. Sci. 94:12457-61 (1997)). A naturally occurring myostatin reduced-function mutation in a human child is associated with gross muscle hypertrophy and a family history of exceptional strength. (Williams M S, N Engl J. Med. 2004 Sep. 2; 351(10):1030-1; author reply 1030-1.)
Additionally, it should be noted that the serum and intramuscular concentrations of immunoreactive myostatin are increased in HIV-infected men with muscle wasting compared with healthy men, and correlate inversely with the fat-free mass index. These data support the hypothesis that myostatin is a negative regulator of skeletal muscle growth in adult men and contributes to muscle wasting in HIV-infected men (Nestor et al., supra).
In view of the above findings, a need exists for a manner of regulating myostatin activity, particularly in individuals who experience muscle wasting as a result of a condition or disease state such as, for example, aging, Autoimmune Deficiency Syndrome (AIDS), Multiple Sclerosis, and cancer. The present invention provides methods and compositions which may be utilized to help individuals with such muscle wasting conditions and provides further insight into the regulation of myostatin gene expression.